Spanner Jaw Sizes

This page has a useful chart for comparing spanner sizes (a wrench
in the USA) and common nut/bolt use. Although it is believed to be
correct, use the suggestions and data provided on this web page at
your own risk! Please read the disclaimer
first. From here you can just jump directly to the Jaw Size Table but I think you should
read the whole page at least once for advice (or twice if you are a
judge for the "most boring web page" competition). Here is the table
of contents:

Background

For the 'British' sizes, the nut/bolt size corresponds to the
spanner marking (i.e. a 1/2" W spanner fits a Whitworth bolt with a
shank/thread diameter of 1/2"). The original Whitworth sizes were
standardised in 1841 by Sir Joseph Whitworth (1803-87) and featured
a significant head oversize to accommodate the crude tolerances of
the production methods of the day. In 1908 the standard was revised
to include the option for a finer thread and became BSF (British
Standard Fine) and BSW (British Standard Whitworth).

During the Second World War the standards were revised again as "War
Emergency B.S. 916 : 1940" as an austerity measure to reduce steel
consumption and this resulted in the head sizes being reduced by one
step (so the head size for the older Whitworth 1/4" bolt was used
for the revised BSW/BSF 5/16" bolt, etc, avoiding the need for new
tools), also bringing them closer to the size range commonly used
today:

Thus all but the oldest
(pre-WW2) 'Whitworth' nuts/bolts are likely to use the BSF/BSW head
size (sometime just marked as 'BS'). As a result, you will sometimes
see spanners marked along the lines of '1/4 W 5/16 BS' indicating
the jaws are sized for a 1/4" Whitworth bolt, or the next step up at
5/16" for BSF/BSW. The BSW bolts/nuts use the same coarse thread as
the original Whitworth, which is suited to soft or coarsely
crystalline materials (e.g. aluminium, cast iron), while the BSF
bolts have the same thread profile but a finer cut (i.e. higher TPI
value) and, with an adequate length of thread engagement, provide a
stronger fastener and better vibration resistance for high tensile
materials.

Typically you will find BSF/BSW in use in British equipment designed
before 1948 (and of course equipment using parts or sub-assemblies
designed in that era, for example, some Land Rover gearboxes), or
similar equipment from members of the former British empire
(commonwealth) such as Australia. One notable continued use of
Whitworth's standard is the camera tripod mount, typically they used
the 1/4" BSW thread (3/8" is on larger cameras), but recently UNC
has replaced this in a rather unsporting move by the standards
bodies.

The BA (British
Association) sizes were formulated in 1884 and standardised in
1903. Later it was recommended to use them for all sizes below 1/4"
instead of BSW/BSF. They are mostly used in electrical and
instrumentation applications and continued in common use in the UK
more or less until metrication in the 1970s when its use started to
decline. Although odd numbered BA sizes have been made and are
listed, they are quite rare. In the UK the even number sizes
from 0BA down to 8BA are still readily available from electronic
component suppliers such as RS
Components, but the smaller or odd number sizes are often only
available from model-making suppliers and companies offering parts
for restoration work.

The AF sizes (UNF/UNC for Unified National Fine/Coarse) are seen
mostly in USA equipment and cars, and British stuff designed from
post-1948 (when it was decided to drop BSW/BSF as the preferred
series of fasteners) up to metrication a couple of decades ago (say
end of the 1970s). There is a 'heavy series' of nuts that have
slightly larger AF sizes for a given bolt size, this table just
shows the common ones. For smaller bolts they use 'numbers' rather
than explicit sizes, but only the even numbers are in common use.
Recently, even the USA has made significant moves towards metric
sizes due to their international adoption.

The metric nut/bolt sizes listed are the common ISO sizes of the
1985 era (mostly same as the DIN standard), but there have been
ISO/ANSI recommendations for some head sizes to be reduced from 1992
onwards, but not everyone followed. Therefore you may find M10 with
16mm AF size instead of 17mm (or possibly even 14mm or 15mm), M12
with 18mm, and M14 with 21mm. The 'non-preferred' sizes are still
quite common, so don't be surprised to find you need such spanners,
and occasionally (e.g. older Citroen cars!) you find oddities like
M7 used. The M3.5 size is not listed in my 1985 copy of "Industrial
Fasteners" but is very commonly used in UK electrical installation
work (e.g. light switches, etc) presumably due to its closeness to
the older 4BA size.

Postscript: It is a common
view that the odd Whitworth/BS spanner sizes are 'illogical' and
that the newer metric system is much more 'logical', but consider
this: To cover 1/4" to 3/4" in both fine (BSF) and coarse (BSW)
threads, the British system defines 9 diameters and requires 9
spanners (or 10 if you also include the original large head
Whitworth sizes as well). The UNF/UNC system defines 8 diameters for
the same range but requires 11 spanners. For the broadly equivalent
range in metric of M6 to M18 using all the common
metric
standards in use, there are 8 diameters defined but a total of
15 spanners are required! (i.e. 69% more spanners per diameters than
the British system). Is this progress?[top]

Using the Table

The table was created from a spreadsheet. The white cells are for AF
sizes, they were entered as the fraction (e.g. "=5/16" in an EXCEL
formula) and printed to 3 decimal places (so 5/16 = 0.3125 = 0.313
when rounded for display), the exact value was then converted to
metric by multiplying by 25.4 and then printed to 2 decimal places
(so 0.3125 * 25.4 = 7.9375 = 7.94 when rounded). The light
blue/green cells have the metric size defined, and the inch size
computed by dividing by 25.4 and rounding to 3 decimal places (e.g.
8mm / 25.4 = 0.314960... = 0.315 rounded). The yellow cells are like
the white ones, except the fractional inch value is entered as the 3
decimal place value from tables of BA/Whitworth data, then converted
as per AF sizes.

The table has a 'Diff' column for showing the difference in jaw size
between that row and the previous row, computed from the 'exact' mm
values then rounded to 2 decimal places (so they may not always
match the result of subtracting the rounded values). If this is
small (e.g. around 0.2mm, but depending on the overall size of
course) you will probably find either spanner will do. If the
difference is really
small, it is probably just rounding from the same figure taken from
different sources or happy coincidence (e.g. 27mm and 1-1/16" AF).

What ever you do, select a spanner or socket that fits properly and
use a 'reasonable' torque for the application. Most critical bolts
(e.g. engine cylinder head) will have a torque setting, tightening
sequence, and possibly a final tightening angle specified to reach
the correct pre-load force - use this information! Also use any
specified tightening sequence in reverse for loosening any bolts to
avoid warping the assembly.[top]

Selecting Spanners

My own choice of spanner is usually the Abingdon King Dick
brand, as they offer quality tools and a good range of British
sizes, but there are others that are good in my own experience, for
example, Beta
Tools (Italy), Stahlwille
(Germany) or Snap-On (USA,
though more expensive). I also have some Britool spanners, which
were good but the holding company (Facom) closed the UK factory
around 2000. I think the brand lives on, however, I don't have any
experience of them since then and suspect the old 'Made in England'
quality has gone.

Of course, quality is not usually cheap in any line of work, but
some on-line prices are not bad. For example, Abbot Tools, A & R Sheldon, or Lawson
HIS are worth trying for King Dick stuff (and other tools). It
is worth checking out all 3 site's prices, and take in to account
postage cost and any large-order discounts they offer.

If you are buying a tool set without any significant existing
collection, and are planning on working on new equipment, then my
suggestion it to get a comprehensive metric set (certainly the ISO
bolt sizes) covering the range you expect (e.g. smaller sizes are
typically needed for a motorbike than a car, etc) and remember that
sizes above 32mm or so become more expensive so it is probably best
to buy them as needed. Then use the table to add any odd sizes you
expect to find occasionally.

As a car tool kit, you should check the sizes you need for a number
of key items: wheel nuts/bolts, battery fixing nuts/bolts, battery
terminals, alternator fixing & adjusting nuts/bolts, and spark
plug size. Note that a 3/8" square drive set is generally not strong
enough for rusted or over-tightened wheel nuts, so 1/2" drive is the
suggested minimum for that job (or use a dedicated single
wheel nut wrench or the 'spider'
style of wheel brace).

For small electrical/electronic work, it makes sense to buy a set of
open ended spanners covering 5, 5.5, 6, 7, 8 and 10mm and a 1/4"
drive socket set covering this range (e.g. King Dick TKS424). A 1/4"
square drive to hex bit holder, and set of hex (Allen key) and Torx
bits to go with this is also a good idea. Alternatively, buying 'nut
drivers' (also known as a 'nut spinners', they look like
screwdrivers but have a hex socket end) or a box spanner set in
place of the socket set can be a cost effective solution.

In the UK it is useful to add a few more, for example, having
combination spanners for 2BA and 4BA for older electrical systems
(and near equivalents, e.g. 4BA covers for 1/4" AF), plus a few of
the common AF sizes (or buy near metric equivalents, such as 11mm
for 7/16" AF and 16mm for 5/8" AF) and/or any BSF sizes depending on
what you expect to work on.

Usually a few of the ISO metric spanners will cover common AF sizes
quite well (for example, 8mm for 5/16", 19mm for 3/4", 22mm is often
passable for 7/8", 24mm for 15/16" and 27mm for 1-1/16") but if you
expect to work a lot on AF based equipment, then get the proper
sizes where your metric selection are not very close (e.g. 3/8",
1/2", 9/16", 11/16" and 13/16" AF as a start). The same applies for
British sizes, some are well covered by metric (e.g. 18mm for 3/8W
and 21mm for 7/16W) but others are best bought if you need them. If
fact, some of the metric sizes that are available seem to exist
simply to cover for British sizes (e.g. some companies offer 33mm
which is perfect for 3/4W)!

If only buying spanners, then get an open set and a ring set (e.g.
King Dick TKO8M & TKR8M) so you can hold both nut and bolt. If
you are also buying a good socket set, then combination spanners
(one end open, other end ring) make some cost saving sense as the
open end is often essential (e.g. nut on a pipe, etc) and the ring
end can sometimes fit in where a socket can't reach, so something
like the King Dick TKC10M spanners and TKS682 3/8" drive set is a
comprehensive option for smaller stuff (to 22mm), but I would add at
least a 16mm combination spanner to the TKC10M set. Alternatively
(or in addition, depending on your budget) the TKS883 1/2" drive set
for larger stuff (to 32mm), maybe also adding to the TKC10M set
spanner sizes 27mm, 30mm and 32mm for completeness, or going all the
way and buying the TKO12M and TKR12M open & ring sets.

The larger socket sets, such as 3/4" and 1" square drive, are
normally used for sizes of around 27mm and above. Typically you need
this if you work on large equipment (tractors, lorrys, trains,
ships, etc) or for the largest and/or unusually high torque nuts on
cars (e.g. drive shaft, occasional suspension bolt, etc). Unless you
need it often, it may be better to hire such tools as needed.

Finally, if you really are looking for a minimum size/weight kit,
then probably you should buy combination spanners in the ISO
preferred sizes 8,10,13,17, and 19mm (possibly also 24mm,
alternatively using a list from checking your actual requirements),
together with a good quality 8" or 10" adjustable wrench (e.g.
Bacho, King Dick, or a genuine USA-made Crescent).

Never rely on an adjustable wrench for anything high-torque, you
will just end up damaging yourself and/or the equipment! Consider an
adjustable as your 'last resort' option.

For releasing very tight nuts/bolts you should consider impact
sockets and slogging spanners, available from King Dick and others,
but you may also want to look at a dedicated supplier of impact
tools such as Impact
Socket Supplies Ltd who also provide special and/or very large
tools (such as bi-square sockets for the railway industry, and up to
3.5" square drive) and Deltec
Industries Ltd who make/supply impact sockets (and will do
specials, such as unusually deep ones, for a reasonable price).

For tightening you should consider getting a good quality torque
wrench (or wrenches, depending on the torque range you require),
such as those made by Norbar
or Williams
Superslim.

While any engineer's tool box is likely to have at least one steel
hammer, it is also a very good idea to have a copper-faced hammer
(and also a hide or plastic face for striking softer items such as
aluminium castings). A good range is made by Thor Hammers and can be
obtained from various engineering supply places, or directly from
the company.[top]

Safe Use

It is very important to use spanners and sockets, indeed any tools,
correctly for the following reasons:

To avoid injury to yourself or others.

To avoid damaging the equipment you are working on.

To have a long and reliable tool lifetime.

Here are some obvious (and less obvious) do's and dont's (and please
do not pedantically read the "don't + never" type of statement as a
positive endorsement!):

DO: Avoid
overreaching yourself and consider the consequences of the spanner
slipping, or the fastener suddenly breaking or coming loose. If the
result is likely to involve a fall, or the removal of flesh from
your knuckles, then reconsider your approach!

DO: Use the
correct fitting tool. Excess play leads to a greater chance of
damage to the fastener or possibly the tool. A very rough guide to
maximum acceptable free play for a spanner or socket ranges from
around 0.2mm to 1mm from the smallest to largest size in the table.

DO: Use high
quality tools, they will cost less in the long run since they should
last longer and lead to less chance of damage or injury. You need
not always buy the most expensive choice, but if your tools set
costs much the same as a child's toy, what do you expect?

DO:
Regularly inspect tools for any signs of wear or damage. Repair (if
possible, e.g. ratchet mechanism) or replace any that are not in
good condition. Periodically verify (and/or return for proper
calibration) any measuring tools such as torque wrenches.

DO: Look
after your tools. Clean them after use and store in a dry location,
preferably in a proper lockable tool box. A quick wipe with a clean
rag and some very light oil (e.g. WD40) will help remove any dirt,
rust, and moisture, but make sure they are not left in a slippery
condition.

If you have high quality chrome plated tools, it is a good idea to
initially and then occasionally
polish them with automotive metal polish such as Autoglym Metal Polish,
or the less abrasive silicone paintwork polish for more regular use.
As well as cleaning the surface, the polish typically leaves a water
resistant residue that helps prevent rust.

DO: Always
use eye protection with any striking or cutting tool (hammer, impact
wrench, nut splitter, etc) or when working in a location where
falling dirt could enter the eyes (e.g. when under a car).

DO: Assemble
fasteners properly, typically making sure they are clean and maybe
lightly lubricated with normal oil (e.g. "3-in-1" or light engine
oil). Follow any recommendations from the maintenance manual about
the correct procedure. In particular, it is important to lubricate
stainless steel fasteners to prevent galling, though
also you should consider any contamination issues (e.g. in food
processing machinery) and use an approved lubricant in those cases.

DO: Make
sure fasteners do not loosen unexpectedly due to vibration or
thermal cycling. For high stress bolts this is normally OK due to
the force when correctly tightened, provided the thickness of the
clamped object is greater than the bolt diameter (so there is enough
'spring' action). In other cases where high forces are
inappropriate, where creep can be expected (e.g. paint coated metal
sheet), or the consequences of failure are serious, then use some
form of positive retention: For example, Nylock nuts (or the
superior Aerotight
type with its PDF
data sheet), spring washers (or the superior Nord-Lock
design), castled nut & split-pin, thread lock compound, etc.

DO: Use a
torque wrench for tightening any critical fasteners where the
correct figure is specified. However, never assume the standard
tables of torque values are the correct ones for any job. They
represent the maximum
for a given fastener size, material and lubrication state, and do
not take in to account the stress limits of the object(s) being
clamped by the fastener. Use any manufacturer recommended values
that are available, and carefully follow any special sequence and/or
final tightening angle.

DON'T: Do
not mix fasteners from different standards (e.g. using UNC nut with
BSW bolt, etc). Even though quite a few sizes have the same outside
diameter and TPI values, the difference in thread angles (e.g. 60°
for UNC and 55° for BSW) results in a serious reduction in strength.
If in doubt, check with a proper thread gauge that you are using the
correct replacement part. Note: Some suppliers of "BSW" fasteners
are actually offering UNC as it is difficult to find the real stuff
these days, but there are still some BSW/BSF manufacturers
and BSW/BSF
distributors around.

DON'T: Do
not replace high tensile steel fasteners with the common stainless
types (e.g. A2-70 grade) unless the application allows you to safely
adjust the tightening torque downwards to match. As an
approximation, A2-70 is equivalent to 7.6 in the typical metric
designation, so the yield strength of around 450N/mm2 is
only 70% of the most common 8.8 'high tensile' grade used for bolts,
and only 38% of the 12.9 grade used for the better quality (normally
black finish) socket cap screws. Unmarked or very cheap 'A2'
stainless fasteners may be equivalent to 5.4 grade, so only 33% of
the strength of the common 8.8 'high tensile' steel, and basically
not much stronger than mild steel (of course, better corrosion
resistance). If both strength and corrosion resistant are needed,
there are special quality stainless fasteners available from Bumax.

DON'T:
Avoid using the wrong type of lubricant when assembling nuts and
bolts.

While light oil is usually advantageous, avoid using any extreme
pressure lubricant (e.g. gearbox oil, normal lubricating grease,
molybdenum disulphide paste, etc) unless this is specifically
required. Doing so significantly increases the clamping force of the
bolt for a given torque, a factor that needs to be taken in to
account for correct tightening. Please note that such extreme
pressure lubricants can be a very good thing to ensure safe and
consistent tightening, and to inhibit corrosion, but the torque used
has to match. Also check that the lubricant is suitable for the
intended environment, not just the operating temperature range, but
also any contamination issues or other side effects (e.g. never use
copper-based materials where acetylene exposure is expected).

I have seen the suggestion of reducing the common (dry, plain steel)
torque table values by 10% for engine oil lubrication, 20% for
anti-size paste (e.g. Copaslip
style of "copper grease"), and 30-50% for molybdenum disulphide
based lubrication paste or other extreme pressure lubricants (e.g.
with graphite, PTFE, etc). For a selection of common cases using
their lock washers Nord-Lock provide a better example of torque
guidelines.DON'T:
Never use a torque wrench as a long lever to release a rusted or
over-tightened fastener! A torque wrench is a precision tool
intended for correct tightening. Instead, use a T-bar or 'jointed
nut spinner' for such high torque releasing jobs, or consider using
an impact wrench or slogging spanner.

DON'T:
Never strike a normal spanner, there are proper "slogging spanners"
for that type of job.

DON'T:
Never, ever, strike a ratchet driver. If you need more than decent
'single hand' force on the ratchet handle, you should consider using
a longer T-bar or breaker bar (also known as a 'jointed nut spinner'
e.g. King Dick SNS208X for 1/2" square drive), or possibly a larger
size of square drive socket set (e.g. 3/4" as the next size up, etc)
if this is a regular problem.

DON'T:
Never use a normal socket with an impact driver. Always use to
proper type of impact socket (they have thicker walls and normally a
black phosphate finish).DON'T: Do not
use 'extension tubes' on spanners or socket set drivers without
considering the maximum sensible force. Avoid using an extender on a
ratchet handle for high torques, as this will greatly accelerate
wear on the mechanism. As an approximate guide, the table below
offers some sensible limits on use based on the following
assumptions:

A typical adult (in good health and fitness, and from a safe
position) can exert a sensible controlled maximum force of 200N
per hand (i.e. just lifting they usual 20kg maximum suitcase
weight for an aeroplane with one hand). Clearly someone with
above average strength (i.e. a job with lots of heavy work, or
regular gym attendance, etc) can do more, but this is already
above the Health and Safety recommendations of a 20kg limit for
carrying with both hands under good position conditions.

The common torque wrench/driver ranges are within the sensible
limits for each socket set's drive capacity, and where this is
lower than the ISO3315 test limits for hand accessories, the
lower value should be used.

The equivalent nut/bolt sizes are based on torque wrench
tightening of an unlubricated metric coarse thread in the most
common grade 8.8 of 'high tensile' steel (equivalent to 'S'
grade in Imperial standards).

These lead to the following guide lines:

Square
Drive
Size

Torque
Tool
(Nm)

Hand Tool
Breaker / T-handle
(Nm to ISO3315)

Biggest
Bolt/Nut

Socket
Size
(mm)

Bolt/Nut
Torque
(Nm)

Length of
lever to centre of
hand(s) for lower torque limit

1/4"

25

62 / 55

M8

13

25

12cm (one hand)

3/8"

110

202 / 180

M12

19

87

55cm (one hand)

1/2"

350

512 / 455

M18

27

294

88cm (two hands)

3/4"

1500

1412 / 1255

M30

46

1440

313cm (two hands)

1"

4500

2515 / 2236

M42

65

4048[*]

559cm (two hands)

[*] This size and grade of bolt cannot be fully tightened using a
"hand tool" so would need a suitable torque wrench or multiplier
gearbox.
Notes: Snap-On do not provide maximum torque limits due to the USA's
national pastime of litigation, so they have some detailed guidelines that suggest working
torque limits of 37, 108, 318, 962 and 1586Nm respectively for the
drive sizes listed to offer long life. Stahlwille indicated to me
that their key 3/4" drive accessories are good for occasional use to
the torque wrench limit of 1500Nm, but 1900Nm was a critical limit.
Beta indicated their official limits are the ISO3315 values. King
Dick informed me they meet the DIN899 standard and the 3/4" drive is
usable up to 1569Nm. Finally, the ISO standard also requires ratchet
handles to match the torque limits for breaker bars (referred to in
ISO3315 as 'flex head nut spinner') but the endurance test for such
items is conducted at much lower torques (e.g. 50,000 cycles at
128Nm for 1/2" drive).

The implications for sensible work are:

Extenders should not be used on spanners below around 15-17mm.

With larger spanners extenders can be used if really
necessary, but do so with care, as it is easy to slip off due to
the large tube needed to clear the opposite head from the one in
use. Preferably do this only on ring spanners for maximum
strength, and least chance of slippage or fastener damage.

The 1/4" drive set should not be used with extenders, nor with
a converter and 3/8" drive tools in order to increase torque.

The 3/8" drive tools should not, as a rule, be used with
extenders or high forces if a converter and long 1/2" drive
handle is in use.

The 1/2" drive tools can be used with modest extenders on a
breaker bar or T-handle to around 50-100cm depending on the care
taken and need for maximum user effort. For example, a longer
handle is reasonable if you cannot stand in a position to safely
use full force of both hands.

The 3/4" drive tools can be used with fairly long extenders,
since there is no way to achieve the needed torque without a
long lever (or a torque
multiplier gearbox). Again, use a length of pipe extension
only on suitable tools such as a T-handle or breaker bar, and if
you appear to need a very long extender then consider getting a
torque multiplier gearbox instead. Then think
about what the reaction foot will act against, as sheet metal
tears quite easily!

Once you are using 1" drive at its full (torque wrench)
capacity, the thought of needing a 1125cm lever (yes, that is
greater than 36 feet long!) is just silly, if not plain
dangerous. Even the limit for extending a T-handle to 559cm (18
feet!) is inconvenient in most cases. At this point the only sensible
option is a torque multiplier gearbox, or dedicated
gearbox and driver tool such as the Norbar 'Pneutorque PT4500'
model or similar.

Finally, if you are of the "7 stone weakling" body build, the
table suggestions should be safe, but if you are of the "300lb
gorilla" body build, then take extra care not to exceed safe
limits! Note that common 3/4" torque wrenches for 1500Nm are
only 120-140cm lever length, suggesting much
more than 200N per hand is possible. (Or of course, more hands,
claws, or tentacles, depending on your species and/or the lunar
cycle...)

DON'T:
Never use a ring spanner (or similar) on one jaw of an open ended
spanner as a makeshift 'extender'. It is very likely to slip
(leading to injury or damage) and can also damage the open jaw,
leading to more problems and expense later on. If you need to extend
a spanner, which you should think twice about anyway, then use a
length of pipe that will apply the force to both to the central bar
(i.e. the normal 'handle' area) and to the widest part of the head
(i.e. the strongest area) of an open jaw wrench.

DON'T: Do
not apply excessive force when undoing fasteners, as that can break
them. Use care and some 'common sense' when releasing or tightening
fasteners. The use of penetrating oil (e.g. 'PlusGas') helps, and
sometimes it is better to occasionally re-tighten a fastener during
a difficult removal to clear the build up of rust or dirt in the
threads. For a novice engineer, get a few small spare low tensile
steel screws & nuts (e.g. M3, M5 and M8) and try breaking them
by over-tightening to get a 'feel' for safe torque levels for such
fasteners before
tackling a real project...[top]

Jaw
Size Table

Here is the chart for comparing spanner sizes and common nut/bolt
use. Not listed are the sizes below M1.6/10BA or above M39 as they
are quite rare to encounter in most folk's life, neither are some of
the very smallest BSW sizes that were generally dropped in
preference for BA sizes at the beginning of the 20th century (for
example the 5/32" screws used in the British made Meccano toy sets
until the 1970's).

The "jaw size" given is often the maximum tolerance of the
across-flats nut dimension, so the actual spanner jaw is likely to
be slightly larger to allow for an acceptable fit.[top]

Acknowledgements

I would like to thank Laurie Barlow for a personal communication on
the BA spanner size, and Dave Reeves for a personal communication
covering the WW2 austerity measures for BSW/BSF head size and
providing excerpts from his copy of "British Standards for Workshop
Practice", Third Impression May 1945.

I would also like to note my thanks to the late Ron Bingham for his
personal communication on the Whitworth sizes. He pointed out that
the original (pre-1940) standards almost
follows the formula AF = 1.5 * bolt diameter + 0.15", but not quite.
It would be nice to know just how and why those odd spanner sizes
for BSW/BSF were actually chosen, the above history sections suggest
is was simply the sizes of common hex bar at the time, but Ron's
observation suggests a little more than this.

Thanks also to Nigel Graham for his comments and feedback.

I am grateful to Tony Pay for his interest & learned advice, and
his suggested tag line for the disclaimer.

Disclaimer (careless torque costs
lives)

The information on this page, while
believed to be correct, is provided with absolutely no warranty
at all. We accept no liability for
any errors or omissions.

Before you use any of the information provided by this page, or its
links, for anything important check with the relevant official
standards first or obtain specialist advice. Errors can be costly
and potentially fatal, as this
incredible example illustrates.